Disk drives are digital data storage devices which may allow host computers to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive may include a plurality of magnetic recording disks which are mounted to a rotatable hub of a spindle motor and rotated at a high speed. Information may be stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information may be written to and/or read from a storage surface(s) of a disk by a transducer or head. The head may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The head may be mounted on an actuator arm that may be capable of moving the head radially over the disk. Accordingly, the movement of the actuator arm may allow the head to access different data tracks. The disk may be rotated by the spindle motor at a relatively high speed, which may allow the head to access different sectors within each track on the disk.
The actuator arm may be coupled to a motor or coarse actuator, such as a voice coil motor (VCM), to move the actuator arm such that the head moves radially over the disk. Operation of the coarse actuator may be controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the head is moved from an initial position to a target track position. A seek may consist of an acceleration stage, a deceleration stage, and a settle stage. In general, the seek function may be initiated when a host computer associated with the disk drive issues a seek command to read data from or write data to a target track on the disk.
As the head approaches the target track, the servo control system may initiate the settle stage to bring the head to rest over the target track within a selected settle threshold or “window”, which may be based on a percentage of the track width from the center of the track. An algorithm may be employed during settle to ensure the head is positioned on the target track with sufficient accuracy to write. This process may typically require counting servo position samples occurring within the settle window. For example, a write gate may be enabled after 10 consecutive positioning samples are observed within a window of +/−10 percent of a data track. A wide variety of settle criteria may be employed, and may take into account a tradeoff between seek time and positioning accuracy. Thereafter, the servo control system may enter the track following mode, where the head is maintained at a desired position with respect to the target track (e.g., over a centerline of the track) until desired data transfers are complete and another seek is performed.
Large off-track write events may lead to unrecoverable errors, also referred to herein as “hard” errors. A write fault gate may be used to terminate write events that exceed a predetermined write fault limit or threshold value. The write fault threshold may be based on a predetermined radial distance between the head and the centerline of a track, which may be determined via tolerance analysis of head parameters, positioning accuracy (for example, based on track misregistration), and/or estimates of overshoot. “Overshoot” may refer to the amount by which an off-track event exceeds the write fault threshold value. The overshoot value may depend on the acceleration and/or velocity of the head.
However, because sampled servo systems may receive position data at fixed time intervals based on the servo position samples on the disk surface, an off-track write event may exceed the write fault threshold value during a time between samples such that the write fault gate may not immediately terminate the write event. Such an off-track write event may be referred to as a write-fault-gate-plus-overshoot (WFGPO) event. Position/velocity/acceleration algorithms may be used to detect the occurrence and/or magnitudes of such write events, for example, based on pre-fault and post-fault position, velocity, and/or acceleration data for the head. However, if the head moves off-track by more than a predetermined amount of overshoot while writing a track, damaging encroachment on adjacent tracks may occur.
In particular, large WFGPO events may occur shortly after a seek event. More particularly, structural resonances of the head stack assembly (HSA) may be excited by the seek current, which may cause transient vibrations during the settle stage. Also, transient seek dynamics may cause rapid positioning error events to occur shortly after a settle. For example, for three adjacent tracks A, B, and C, a worst case may occur if middle track B experiences a WFGPO event from both outer track A and track C, and the events are circumferentially aligned such that a single data sector of track B is encroached or “squeezed” from both sides. This may be referred to as a double-sided squeeze event. Some read/write systems may tolerate about a 20%-25% double-sided squeeze and/or about a 40%-50% single-sided squeeze before realizing a hard error. Furthermore, as track pitch decreases (i.e., as tracks-per-inch (tpi) increases), the read/write system may become more susceptible to off-track write events because the magnitude of the WFGPO events may not necessarily decrease in proportion with the track pitch.